The demand for progressively smaller, less expensive, and more powerful electronic products creates a need for smaller geometry integrated circuits (ICs) and larger substrates. It also creates a demand for a denser packaging of circuits onto IC substrates. The desire for smaller geometry IC circuits requires that the dimensions of interconnections between the components and the dielectric layers be as small as possible. Therefore, recent research continues to focus on the use of low resistance materials (e.g., copper) in conjunction with insulating materials with low dielectric constant (k) between the metal lines.
The use of low resistance materials is needed because of the reduction in the cross sectional area of via interconnects and connecting lines. The conductivity of interconnects is reduced as the surface area of interconnects is reduced, and the resulting increase in interconnect resistively has become an obstacle in IC design. Conductors having high resistivity create conduction paths with high impedance and large propagation delays. These problems result in unreliable signal timing, unreliable voltage levels, and lengthy signal delays between components in the IC. Propagation discontinuities also result from intersecting conduction surfaces that are poorly connected and from the joining of conductors having highly different resistivity characteristics.
There is a need for low resistivity interconnects and vias that have the ability to withstand volatile process environments. Aluminum and tungsten metals are often used in the production of integrated circuits for making interconnections or vias between electrically active areas. These metals have been used for a long time in the production environment because the processing technologies for these metals were available. Much experience and expertise with these metals have been acquired.
Copper is a natural choice to replace aluminum in the effort to reduce the size of lines and vias in an electrical circuit. The conductivity of copper is approximately twice that of aluminum and over three times that of tungsten. As a result, the same current can be carried through a copper line having half the width of an aluminum line.
However, there have been problems associated with the use of copper in IC processing. Copper poisons the active area of silicon devices, creating unpredictable responses. Copper also diffuses easily through many materials used in IC processes and, therefore, care must be taken to keep copper from migrating.
Various means have been suggested to deal with the problem of copper diffusion into integrated circuit materials. Several materials, including metals and metal alloys, have been suggested for use as barriers to prevent copper diffusion. The typical conductive diffusion barrier materials are TiN, TaN and WN. Addition of silicon into these materials to create TiSiN, TaSiN, and WsiN, respectively, could offer improvements in the diffusion barrier properties. Silicon nitride has been the best non-conductive diffusion barrier material so far.
Diffusion barrier materials could be deposited by the chemical vapor deposition technique. For example, in the case of TiN CVD deposition, a precursor that contains Ti and optionally nitrogen is used. The precursor decomposes at the selected surfaces, and the decomposed elements react together to form a TiN layer on the selected surfaces. Reaction by-products (i.e., products produced by the precursor decomposition and the following reactions that do not deposited on the selected surfaces) are often volatile and are exhausted away.
Of equal importance with the use of low resistance materials in interconnecting lines is the introduction of low dielectric constant materials (low-k dielectrics) for insulating between the interconnecting lines. Low k dielectrics are insulating dielectric materials that exhibit dielectric constants that are less than those of conventional IC dielectric materials such as silicon dioxide (k value of about 4), silicon nitride (k value of about 7), and silicon oxynitride (k value of about between 4 and 7).
Various low-k dielectrics have been introduced including fluorine doped silicon dioxide (k value of about 3-3.6), carbon doped silicon dioxide (k value of about 2.5-3.3), fluorinate carbon (k value of about 2.5-3.6), and organic materials such as parylene (k value of about 3.8-3.6) and polyimide (k value of about 3-3.7). Some of these materials have been successfully incorporated into the IC fabrication processes, but others have not been because of various difficulties involved with the integration. The low k dielectrics can be deposited by CVD or spin-on techniques.
Further research is focusing on porous low-k dielectrics because of their potential lower dielectric constants (2-3). Examples of porous low dielectric materials are porous hydrosilsesquioxane or porous methyl silsesquioxane, porous silica structures such as aerogel, low temperature deposited silicon carbon films, low temperature deposited Si—O—C films, and methyl doped porous silica.
The use of porous low-k dielectrics presents significant integration problems such as low mechanical strength, poor dimensional stability, poor temperature stability, high moisture absorption, permeation, poor adhesion, large thermal expansion coefficient, and unstable stress level.
Of the various problems associated with porous low-k dielectrics, the trapping of small molecules in porous low-k dielectrics is one that is recognized in IC processes. U.S. Pat. No. 6,417,118 to Hu et al. discloses a method to prevent further absorption of moisture into a porous low-k dielectric film by treating the porous film with a reactive solution to convert the porous low-k dielectric surface from the hydrophillic state (attracting moisture) to the hydrophobic state (repelling moisture) after all the trapped moisture was removed by low temperature annealing. U.S. Pat. No. 6,486,061 to Xia et al. discloses a method for providing a dielectric film having enhanced adhesion and stability that uses post deposition treatment that densifies the film in a reducing environment such as NH3 or H2. By post deposition annealing in NH3 or H2, Xia et al. found that the dielectric film becomes more moisture resistant and retains a low value of dielectric constant even when exposed to the ambient for a week.
The integration of porous low-k dielectrics remains a problem. Even with treatments of low-k dielectric films, the adhesion of the subsequent film, such as a diffusion barrier film for copper interconnect, remains problematic. Since the subsequently deposited films are often impermeable to the trapped molecules such as moisture, alcohol vapor, HCl vapor, and HF vapor, the release of these trapped molecules can cause delamination that leads to device failure.